The word “printer” as used herein encompasses any apparatus, such as a digital copier, book marking machine, facsimile machine, multi-function machine, etc., that produces an image with a colorant on recording media for any purpose. Printers that form an image on an image receiving member and then transfer the image to recording media are referenced in this document as indirect printers. Indirect printers typically use intermediate transfer, transfix, or transfuse members to facilitate the transfer and, in the case of transfix and transfuse members, fusing of the image from the image receiving member to the recording media. In general, such printing systems typically include a colorant applicator, such as a printhead, that forms an image with colorant on the image receiving member. Recording medium is fed into a nip formed between the surface of the image receiving member and a transfix roller to enable the image to be transferred and fixed to the print medium so the image receiving member can be used for formation of another image.
A schematic diagram for a typical indirect printer that includes a printhead that ejects phase change ink on the image receiving member to form an image on the member is illustrated in FIG. 8. The solid ink imaging device, hereafter simply referred to as a printer 110, has an ink loader 112 that receives and stages solid ink sticks. The ink sticks progress through a feed channel of the loader 112 until they reach an ink melt unit 114. The ink melt unit 114 heats the portion of an ink stick impinging on the ink melt unit 114 to a temperature at which the ink stick melts. The liquefied ink is supplied to one or more printheads 116 by gravity, pump action, or both. Printer controller 122 uses image data to be reproduced on media to control the printheads 116 and eject ink onto a rotating print drum or image receiving member 140 as image pixels to form an ink image. Recording media 120, such as paper or other recording substrates, are fed from a sheet feeder 118 to a position where the ink image on the image receiving member 140 can be transferred to the media. To facilitate the image transfer process, the media 120 are fed into a nip between the transfer, sometimes called transfix roller 150, and the rotating image receiving member 140. In the nip, the transfix roller 150 presses the media 120 against the image receiving member 140. An assembly 124 of lever arms, camshafts, cams, and gears urged into motion by an electrical motor responds to signals from the controller 122 to move the transfix roller into and out of engagement with the image receiving member 140. Indirect or offset printing refers to a process, such as the one just described, of generating an ink or toner image on an intermediate member and then transferring the image onto some recording media or another member.
To optimize image resolution in an indirect printer, the conditions within the nip are carefully controlled. The transferred ink drops should spread out to cover a specific area to preserve image resolution. Too little spreading leaves gaps between the ink drops while too much spreading results in intermingling of the ink drops. Additionally, the nip conditions are controlled to maximize the transfer of ink drops from the image member to the print medium without compromising the spread of the ink drops on the print medium. Moreover, the ink drops should be pressed into the paper with sufficient pressure to prevent their inadvertent removal by abrasion thereby optimizing printed image durability. Thus, the temperature and pressure conditions are important parameters for image quality and need to be carefully controlled throughout the nip.
The image receiving member 140 is a hollow cylinder mounted about a shaft that is supported on its ends by stiff endbells incorporated into the shaft. The shaft of the image receiving member 140 deflects under the pressure of the transfix roller 150 at the nip 144. Some deflection of the image receiving member 140 is inherent. Because the shaft of the image receiving member 140 is supported only at the endbells, it deflects more in the middle than at the ends and, thus, applies more pressure to the nip 144 at the ends than at the middle. However, too much deflection by the image receiving member 140 diminishes the quality of the print because of inconsistencies in the pressure at the nip 144. The thickness of the image receiving member 140 is selected to require as little material as possible to provide desirable thermal properties for the imaging surface, which are described below, and to keep manufacturing costs down. However, the thickness of the image receiving member 140 is also selected so that, under pressure from the transfix roller 150 at the nip 144, it does not deflect so much that it diminishes the quality of the print.
The transfix roller 150 includes a cylinder mounted about a shaft and is formed of steel, or another material with similar properties. As described above with reference to the image receiving member 140, the transfix roller 150 deflects more in the middle than at the ends because it is supported only at the ends. The variation in deflection along the length of the transfix roller 150 results in variation of the pressure along the length of the nip 144. The thickness of the transfix roller 150, like that of the image receiving member 140, is selected to balance material costs with the amount of deflection along the transfix roller 150.
When an indirect printer, such as the one shown in FIG. 8, is powered on, the image receiving member needs to be heated to a predetermined temperature that enables the melted phase change ink to remain on the surface of the image receiving member, yet be malleable enough for transfer and fixing to the recording media when the ink image enters the nip. An image receiving member with a larger thermal mass requires more thermal energy and more time to reach the predetermined temperature than an image receiving member that has a smaller thermal mass. One way to reduce the time required for an image receiving member to reach the predetermined temperature is to reduce the thickness of the wall of the image receiving member. While this reduction in wall thickness does decrease the time required for the image receiving member to reach the predetermined temperature, it also affects the pressure conditions in the nip formed with the transfix roller. Without a change to the transfix roller, the pressure in the nip becomes less uniform and weaker in the center of the nip between the ends of the transfix roller and the image receiving member, especially as the walls of the image receiving member are thinned.
As shown in FIG. 9, a nip formed with an image receiving member having a relatively thick wall (for example, 9 mm) has one pressure profile from one end to the other end of the nip across the width of the transfix roller and image receiving member, while a nip formed with an image receiving member having a relatively thin wall (for example, 4.5 mm) has another profile. Whether a wall is considered relatively thick or thin depends on the length of the image receiving member, the material with which the image receiving member is made, and the level of pressure required in the transfix nip. As used in this document, a “thin wall” refers to a wall of a roller having a thickness that is 5 mm or less, while a “thick wall” refers to a wall of a roller having a thickness that is 8.5 mm or more for an aluminum roller approximately 345 mm long and generating a minimum peak nip pressure of about 7 MPa. The ends of the nip 144 correspond to the ends of the image receiving members 140 and the transfix rollers 150. The pressure profile for the thin wall image receiving member has a pressure at each end of the profile that is greater than the pressure at each end of the profile for the thick wall image receiving member. The pressure is highest at the ends of the nips 144 because the image receiving members 140 and the transfix rollers 150 are supported at the ends and are the most rigid at those areas. Additionally, the pressure in the center of the thin wall image receiving member profile is substantially below the pressure in the center of the thick wall image receiving member profile. The pressure is lowest at the middle of the nips 144 because the image receiving members 140 and the transfix rollers 150 deflect the most at the middle, the area that is the farthest from the supported ends. These pressure differences across the length of the nip can cause wrinkles in the recording media and corresponding print quality defects.
One way to modify the nip conditions to help ensure the print quality is adequate and the media is not distorted with thinner wall image receiving members is to add a crown to the transfix roller. As shown in FIG. 10, a crown 160 is a convex profile formed in the elastomer coat 153 of the transfix roller 150. Accordingly, the diameter 190 of the transfix roller 150 is largest at the middle of the crown 160. The crown 160 provides additional interference to the center of the transfix roller 150, increasing pressure at the center of the nip and compensating for the decreased pressure in the center of the nip generated by the thinner wall of the image receiving member. As the wall of the image receiving member is made thinner, the crown of the transfix roller needs to be larger to compensate for the additional image receiving member deflection. The height of a crown, however, is limited by practical constraints in manufacturing and usage.
Additionally, the height of a crown can generate wrinkles and/or image quality defects when print conditions are particularly likely to form either transverse or longitudinal wrinkles. Longitudinal wrinkles are formed in the print media in a direction parallel to the direction that print media is fed through the nip (also known as the process direction). One print condition that is likely to generate longitudinal wrinkles is the center of the print media moving through the nip at a faster rate than the edges of the print media. This condition can result from a crown that is not high enough to compensate for the greater deflection, and resulting lower pressure, in the center of the nip. This condition can also result from high density, process direction images along the edges of the print. Another condition that is likely to generate longitudinal wrinkles is print media being A3 or a similar size. Another condition that is likely to generate longitudinal wrinkles is the orientation of the paper grain in a direction perpendicular to the direction that the print media is fed through the nip (also known as the cross-process direction). Increasing the pressure applied at the center of the nip reduces the occurrence of longitudinal wrinkles.
Transverse wrinkles are formed in the print media in the cross-process direction. One print condition that is likely to generate transverse wrinkles is the edges of the print media moving through the nip at a faster rate than the center of the print media. This condition can result from a crown that is too high and overcompensates for the deflection, resulting in high pressure, in the center of the nip. This condition can also result from high density, process direction images in the center of the print or over the entire print. Another condition that is likely to generate transverse wrinkles is the print media being A3 or a similar size. Another condition that is likely to generate transverse wrinkles is a process direction orientation of the paper grain. Decreasing the pressure applied at the center of the nip reduces the occurrence of transverse wrinkles.
As described above, longitudinal wrinkles and transverse wrinkles can be generated by opposite conditions and, thus be reduced by opposite adjustments. Accordingly, enabling adjustment of the pressure along the nip when print conditions include stresses likely to generate longitudinal or transverse wrinkles is a desirable goal.